We are studying post-receptor light-adaptation and spatial adaptation of human chromatic pathways. Pathways are isolated by Stile's two-color threshold method modified to include spatial and temporal patterns. Three projects are being pursued. 1. Spatial adaptation of the short-wavelength cone pathway is measured with violet or red interference gratings on a yellow field. Spatial adaptation is orientation-selective and is monocular when confined to the short-wavelength pathway. Selectivity for pattern orientation, spatial frequency and temporal frequency is being investigated. 2. Post-receptor light adaptation of "red" or "green" cone pathways is measured with simultaneous incremental and decremental, red and green flashes on a mixed red-and-green field. Certain test combinations are more visible with the addition of a uniform adapting field. The experiments will quantify this post-receptor sensitivity increase in terms of detection levels for many red and green test ratios with several field combinations. 3. Chromatic nature of spatial antagonism is studied with a small diameter chromatic adapting field and a tiny, chromatic superposed test flash. Threshold rise of 2 log units can be produced by dim fields if the stimuli influence only red and green cones. Blue cone signals do not induce red or green saturation. Possible spatial antagonism within the blue cone pathway itself is being examined.